The present invention is described below in particular with reference to so-called hot-film mass air flow sensors, as known, for example, from Konrad Reif (editor): “Sensoren im Kraftfahrzeug [Motor Vehicle Sensors],” 1st edition, 2010, pp 146-148. Such hot-film mass air flow sensors are generally based on a sensor chip, in particular a silicon sensor chip, for example, including a sensor diaphragm as a measuring surface or sensor area, over which the flowing fluid medium may flow. The sensor chip generally includes at least one heating element and at least two temperature sensors which are, for example, situated on the measuring surface of the sensor chip, the one temperature sensor being situated upstream from the heating element and the other temperature sensor being situated downstream from the heating element. It is possible to infer a mass flow rate and/or a volume flow rate of the fluid medium from an asymmetry of the temperature profile detected by the temperature sensors, which is influenced by the flow of the fluid medium.
Hot-film mass air flow sensors are generally designed as plug-in sensors which are able to be fixedly or replaceably introduced into a flow tube. For example, this flow tube may be an intake tract of an internal combustion engine.
A partial flow of the medium flows through at least one main channel provided in the hot-film mass air flow sensor. A bypass channel is formed between the inlet and the outlet of the main channel. In particular, the bypass channel is formed in such a way that it includes a curved section for redirecting the partial flow of the medium which has entered via the inlet of the main channel, the curved section transitioning further along into a section in which the sensor chip is situated. The last-mentioned section constitutes the actual measuring channel in which the sensor chip is situated.
In practice, such hot-film mass air flow sensors must satisfy a variety of requirements. In addition to the goal of minimizing a pressure drop at the hot-film mass air flow sensor overall via suitable fluidic designs, one of the main challenges is to further improve the signal quality and the ruggedness of the devices with respect to contamination by oil and water droplets, as well as sooty particles, dust particles, and other solid particles. This signal quality relates, for example, to a mass flow rate of the medium through the measuring channel leading to the sensor chip, and possibly to minimizing a signal drift and improving the signal-to-noise ratio. The signal drift relates to the deviation, for example, of the mass flow rate of the medium in terms of a change in the characteristic curve relationship between the mass flow rate actually occurring and the signal to be output which is ascertained as part of the calibration during manufacture. When ascertaining the signal-to-noise ratio, the sensor signals which are output in a rapid chronological sequence are considered, whereas the characteristic curve drift or signal drift relates to a change in the average value.
In the case of conventional hot-film mass air flow sensors of the type described, a sensor carrier including a sensor chip which is attached to it or introduced into it generally extends into the measuring channel. For example, the sensor chip may be glued into or onto the sensor carrier. The sensor carrier may, for example, form a unit having a bottom plate made of metal on which an electronic circuit, a control and evaluation circuit (for example, including a circuit carrier, in particular a printed circuit board), may also be glued. For example, the sensor carrier may be designed as an integrally injection-molded plastic part of an electronic module. The sensor chip and the control and evaluation circuit may, for example, be connected to each other via bond connections. The electronic module resulting in this way may, for example, be glued into a sensor housing, and the entire plug-in sensor may be sealed using covers.
German Published Patent Application No. 198 15 656 A1 describes a sensor system for determining at least one parameter of a fluid medium flowing through a channel, in particular an intake air mass of an internal combustion engine. The sensor system includes at least one sensor chip which is situated in the channel for determining the parameter of the fluid medium. The sensor chip is accommodated in a sensor carrier extending into the channel. The channel leads to an outlet opening emptying into a flow tube at an exterior surface of the sensor system. At least one protuberance is provided in the vicinity of the outlet opening on the outer surface of the sensor system having the outlet opening.
German Published Patent Application No. 197 38 337 A1 describes a hot-film mass air flow sensor having a step downstream from the outlet.
In such hot-film mass air flow sensors, a boundary layer forms continuously on the outer surface up to the bypass outlet. The large velocity range which the hot-film mass air flow sensor must cover results in the bypass outlet lying in a laminar, turbulent transition of the boundary layer at certain velocities. This transition area is characterized by poor reproducibility and nonlinear behavior. Thus, this results in a poorer adjustability and a complex behavior in the case of a pulsing flow which includes the transition area.
Therefore, hot-film mass air flow sensors have been developed in which a step having a separation edge is situated upstream and downstream from the bypass outlet, which forms a chimney-like projection. The step in the wall contour forces the regeneration of a turbulent boundary layer. Thus, interactions of the boundary layer at the bypass outlet with a possibly existing transition area upstream are minimized, making a more exact calibration to the target characteristic curve possible.
Despite the numerous advantages of the methods known from the related art for calibrating with the target characteristic curve, they still have potential for improvement with respect to other functional aspects. Thus, the height of the chimney-like projection required for a regeneration of the boundary layer increases with increasing air mass flow rate in the aforementioned related art. This conflicts with requirements for a preferably short sensor housing.